7.2 OSPF for IPv4 - 7.4 Flashcards
In order for routers to become neighbors they must
Share a data link.
Have matching basic OSPF settings, including the OSPF area.
Maintain an exchange of hello packets.
What is a designated router
to act as a central point for exchanging of OSPF information between multiple routers on the same
How is a designated router used in mult-point OSPF
The router with the highest OSPF priority is selected as the DR.
If there is a tie for the highest priority, the router with the highest Router identification (RID) is chosen.
The router with the second highest priority is usually selected as the backup designated router (BDR).
Once the designated router is chosen, all of the non-designated routers exchange link-state data with it, but not with each other.
Describe the down state
When an interface becomes active after being in a down state, the router multicasts an hello packet. Be aware of the following facts about hello packets:
Describe the attempt state
Initially, a router’s list of neighbors has a null value. As soon as it receives an hello packet,
Describe the init state
If all of the information in the hello packet matches the router’s settings, the router indicates to the neighbor that it is in the init state. The router begins to include the new RID in its list of neighbors.
Describe the 2-way state
As soon as a router receives a hello packet that includes its own RID in a list of neighbors, it has reached the 2-way state. When the 2-way state has been reached, routers may exchange link-state databases (LSDB) as follows:
Describe the Exstart
In the exstart state, the routers negotiate how the data transfer will take place and choose the initial sequence number for adjacency.
Describe the Exchange state
In the exchange state, the two routers exchange database descriptions. Each router analyzes the description and determines the information it needs from the other router’s database.
Describe the Loading state
n the loading state, the routers exchange link-state updates (LSUs) that contain link-state advertisements (LSAs). Once the routers exchange this information, they will have matching databases.
Describe the Full state
When the routers have matching databases, they have entered the full state. Full is a stable neighbor state for a multi-point network.
Router(config)#router ospf [process-id]
Enters configuration mode for OSPF. The process ID identifies a separate routing process on the router.
Process IDs do not need to match between routers. In other words, two routers configured with different process IDs can share OSPF information.
Router(config-router)#network a.b.c.d w.w.w.w
area [number]
Identifies networks that participate in OSPF routing:
a.b.c.d is the network address. This can be a subnetted, classless network.
w.w.w.w is the wildcard mask. The wildcard mask identifies the subnet address.
number is the area number in the OSPF topology. The area number must match between routers.
Router(config-router)#router-id a.b.c.d
Configures the router ID for the OSPF process.
Router(config)#interface ethernet0/1
Router(config-if)#ip ospf priority [0-255]
Sets the OSPF priority number for an interface:
The priority number is used in the DR/BDR election process. The router with the highest priority becomes the DR.
Configure a value of 0 to ensure that a router never becomes the DR or BDR.
The priority is set on an interface and applies to the DR/BDR election process on that interface.
